"Overall, growth rates slowed compared to the previous year - a sign that, with almost 2 million members as at the time of this report, the 'low hanging fruit' of the KiwiSaver market has already been plucked," Chaplin said.

"This year the number of providers reporting net member losses also increased markedly with seven in this category, compared to only one in the previous period."

However, Chaplin pointed out that those losses were "typically tiny" and from some of the smallest schemes.

"But the most surprising statistic in the above category is Fidelity's net loss of 2,292 members over the period, equivalent to 3.2 per cent of its total membership," he said.

A core group of providers - NZ Funds, Fisher Funds, KiwiBank and Milford - were all able to expand their membership base by more than 50 per cent in the year.

Westpac recorded annual member growth of 28.5 per cent, compared to the next best scheme, National Bank, which grew membership by 19.6 per cent.

KiwiSaver, now five-years old, had passed out of "the toddler stage" but was "still not a grown-up", Chaplin said.

"Despite a number of high-profile scheme mergers, though, the much-mooted KiwiSaver consolidation process hasn't as yet begun in earnest."

He predicted that could all change in the year ahead.

At Tower's quarterly briefing this week, chief executive Sam Stubbs said the monthly rate of KiwiSaver membership growth had slowed to between 15,000 and 20,000 each month.

That was down from rates of 25,000 to 40,000 new members each month a few years ago.

Stubbs said it was inevitable that we would start to see "the rationalisation of smaller and unprofitable schemes".

Schemes lacking critical mass now had "probably missed the boat".

The Credit Union scheme sold out to Fisher Funds in April and the Gareth Morgan KiwiSaver scheme would soon officially merge with the KiwiBank scheme.

Chaplin said it was to be presumed that, with the disappearance of the National Bank brand, its KiwiSaver scheme would also be reabsorbed by its ANZ parent.

He added that AMP and Axa, who have a single owner, may cease to operate as dual schemes.

Other 'marginal providers' would start to find the going tougher as new governance rules came in to play and tough new reporting standards were due to take effect next year, he said.

Only one new provider, Iwiinvestor, was known to be officially joining the mix in the 2012/13 reporting season.

It was understood BNZ could be launching its new scheme early in the new year, he said.

Chaplin also looked at transfers between KiwiSaver providers.

In the 12-month period, 19 providers had a net loss in the transfer market while 21 gained as a result of transfers.

"As the easy flows from new join-ups inevitably eases, providers will increasingly target rivals in order to grow," he said.

The top five losers - AMP, Axa, Tower, Mercer and OnePath - are all default providers.

ASB was the only default provider to buck the trend, gaining almost $30 million in net transfers.

Fisher Funds came out top of the transfer tables, gaining almost $200 million over the period.

"The result, however, was chiefly due to the Huljich transfer and is likely to be a one-off," Chaplin said.

The report showed that KiwiSaver had collected about $12.8 billion in contributions so far, which amounted to an average of $6736 per member.

About $6.5 billion of total funds were now managed by three Australian banks - ASB, Westpac and ANZ.

Those three providers and AMP/Axa controlled $8.4 billion, which was about 65 per cent of total funds.

A total $135 million was paid in fees in the year, an average of around $70 per member.

Chaplin noted that of the 41 schemes surveyed, just over half achieved a return above the official cash rate of 2.5 per cent.